The present invention relates generally to an improved method and system for measuring position on earth from a moving platform, such as a ship, using signals from the NAVSTAR Global Positioning System satellites, commonly called GPS satellites. In particular, the present invention relates to civilian GPS receivers, that is, receivers which do not utilize knowledge of the potentially unavailable P code component of the GPS signals to determine position information.
Conventional civilian GPS receivers utilize simultaneous pseudorange, in other words, group delay observations of the C/A code components of the L1 band signals received from a plurality of GPS satellites to determine position information. A major source of position errors with such conventional GPS receivers is multipath. Multipath errors may be reduced by time averaging of observations made from a fixed position. Conventional time averaging, however, cannot be used to improve the accuracy of receivers on ships because the resultant position information would relate to the average position of the ship during the observation period, not the instantaneous position.
Position errors also result from ionospheric group delay effects in such pseudorange measurements. The magnitude of the delay encountered by a signal in the ionosphere varies with local conditions and cannot be predicted with sufficient accuracy to be eliminated from position measurements made by GPS receivers. The magnitudes of such errors are frequency dependent, however, and can be determined from simultaneous measurements of signals in different frequency bands. In particular, the GPS system was designed so that simultaneous measurement of signals in the L1 and L2 bands could be used to determine ionospheric delay. This technique is routinely used in military GPS receivers. Conventional civilian receivers measure the C/A code group delay in the L1 band, but cannot make L2 band C/A code group delay measurements because the C/A code modulation is not presently applied to signals transmitted in the L2 band. Simultaneous measurements therefore can not be conveniently made for both L band signals.
In general, conventional civilian receivers are limited in accuracy because of their reliance on group delay, and because of their requirement for knowledge of a code modulating the signals in a GPS band in order to measure the group delay of the signals received in that band.